Abstract: Objective To characterize the etiological spectrum of respiratory outbreaks in the northeastern suburbs of Beijing during the 2017-2024 surveillance period, to elucidate local epidemiological patterns of influenza viruses, and to investigate the molecular evolution of influenza B virus, thereby providing an evidence base for influenza prevention and control. Methods Respiratory specimens were collected from febrile and influenza-like illness cases associated with outbreaks between 2017 and 2024. Influenza viral RNA was detected by real-time quantitative PCR (qRT-PCR), and descriptive epidemiological methods were applied to delineate subtype-specific prevalence. Thirteen BV isolates were randomly selected and their hemagglutinin (HA) genes amplified by reverse-transcription PCR (RT-PCR) and sequenced. Together with homologous BV HA sequences retrieved from the GISAID database, maximum-likelihood phylogenies were constructed to infer genetic characteristics and evolutionary dynamics. Results Among 6 250 outbreak-associated specimens, 3 517 were positive for influenza virus, yielding an overall positivity rate of 56.27%. Across the surveillance period, subtype A/H3N2 predominated (55.90%), followed by A/H1N1 (26.81%), BV (13.14%), and B/Yamagata (4.15%). Co-circulation of A/H1N1 and influenza B viruses was observed in 2017-2018;alternating circulation of A/H1N1, A/H3N2, and BV in 2018-2019;exclusive dominance of A/H3N2 in 2019-2020;overwhelming predominance of BV in 2021-2022;joint dominance of A/H1N1 and A/H3N2 in 2022-2023;and alternating dominance of A/H3N2 and BV in 2023-2024. Phylogenetic analysis revealed that all 13 BV isolates clustered within the V1A clade. Relative to contemporary vaccine strains, outbreak isolates accumulated multiple amino-acid substitutions in critical antigenic regions of the HA protein, including the 120-, 150-, and 160-loops and the 190-helix. Conclusions From 2017 to 2024, influenza outbreaks in northeastern Beijing exhibited considerable subtype heterogeneity. Although BV outbreak strains maintained high overall homology with corresponding vaccine strains, they displayed increasing genetic diversity and acquired mutations at several pivotal HA antigenic sites. Continuous surveillance of both epidemiological trends and molecular evolution is therefore imperative to inform evidence-based respiratory-disease control strategies with emphasis on influenza.

Key words: Influenza B virus, Hemagglutinin gene, Molecular evolution, Influenza-like illness